This invention relates generally to receivers, and more particularly to receivers that receive modulated signals in phase-encoded communications.
Decision Feedback Equalization (DFE) is a technique for quadrature amplitude modulation (QAM) receivers. In DFE receivers, two sections are used to correct for intersymbol interference and gain/phase rotation variations: a feedforward equalizer section which does a linear weighted average of time samples (convolution) to adjust for intersymbol interference caused by following symbols and a feedback equalizer section which adjusts the output of the feedforward equalizer section to remove intersymbol interference from the present estimate caused by previously detected symbols. The feedback equalization section is nonlinear because it contains previously detected symbols.
Commonly a phase shift difference occurs between the phase of a received symbol and a true phase of the current symbol, causing phase noise in an incoming signal of symbols at a receiver. Often this phase shift difference results from instability in the transmitter and/or receiver oscillators and by the Doppler effect. The phase noise has a cumulative effect that shows up as an increasing difference between symbol rotations, if delays between symbols are increasing.
Phase noise may be corrected implicitly by phase-locked loop (PLL) demodulation. For every symbol, an estimation of the current phase shift is generated, based on an average of the phase difference between previously received symbols (“soft decisions”) and corresponding hard decisions.
Thus, there is a continuing need for better ways to reduce phase noise in phase encoded communications signals.